1. Field of the Invention
The present invention relates to an image reading apparatus of an image scanner or the like and a lens mounted to an image reading apparatus.
2. Description of Related Art
A reading apparatus for scanning a draft of a document, an image or the like and reading the draft as a digital image data has been spread. As such a reading apparatus, a copier, a facsimile, an image scanner or a complex machine having all of functions thereof is known. The image reading apparatus provides digital image data accurately reproducing a draft to be read by focusing an image of the draft onto a solid imaging element of CCD or the like by an image reading lens included in the image reading apparatus. Therefore, an image reading lens of an image reading apparatus is requested to have a focusing amplification with small various aberrations balanced and a high resolution. Further, in recent years, as an image reading apparatus is wholly downsized and low in cost, an image reading lens is also requested to be downsized and low in cost.
There is proposed an image reading lens downsized by being constituted from a small number of 4 lenses and maintaining a high resolution. For example, there is known an image reading lens having a sufficiently high resolution by using a lens of a non-rotational symmetry even when constituted by 4 lenses (for example, JP-A-11-190820).
Further, for example, there is known an image reading lens achieving a sufficient resolution even with a compact constitution, in which a positive meniscus lens directing a convex surface toward the object side, a biconcave lens, a biconvex lens, and a negative meniscus lens directing a convex surface toward an image side are arranged in order from the object side and a specific condition is satisfied (for example, JP-A-2002-296499).
Further, for example, there is known an image reading apparatus achieving an excellent optical performance with a wider angle of view, in which a negative meniscus lens directing a concave surface toward the object side, a biconvex lens, a positive meniscus lens directing a convex surface toward the image side, and a negative meniscus lens directing a concave surface toward the object side are arranged in order from the object side and a specific condition is satisfied (for example, JP-A-2007-121743).
However, although the image reading lenses disclosed in JP-A-11-190820 and JP-A-2002-296499 mentioned above achieve a comparative excellent optical performance when a half angle of view is equal to or smaller than about 25 degrees, when the half angle of view is equal to or larger than 25 degrees, a curvature of field is large. Therefore, when the image reading apparatus is further downsized and become higher in resolution, it is difficult to achieve an excellent optical performance over an entire range of a draft to be read.
Further, although in an image reading lens described in JP-A-2007-121743, a curvature of field is excellently corrected even when a half angle of view is equal to or larger than 30 degrees, F-number is as large as 7.0 and the lens is a slow lens. Therefore, in the image reading apparatus using the image reading lens of JP-A-2007-121743, a speed of reading a draft is obliged to be slow, which pose a problem in high speed reading and high resolution.
Further, generally, although in order to realize a fast image reading lens, it is necessary for a lens disposed on the most object side to direct a convex surface toward the object side, there is a harmful influence of increasing a curvature of field. Conversely, when a lens disposed on the most object side is constituted by a lens directing a concave surface toward the object side, the curvature of field can be restrained, but it is difficult to ensure a sufficient speed.